Asthma incidence is increasing, and in adults work-related exposures may be an important factor-occupational asthma (OA) incidence increased 70% over last decade according to a recent registry based study. The true contribution of occupational exposures to adult-onset asthma is unknown because the methods for measuring OA give conflicting results. Methods based on surveillance of clinically diagnosed OA account for <1-5% pf adult-onset asthma. However, case-control methods of measuring asthma risk by industry suggest that 6-33% of adult-onset asthma is caused by workplace exposures. The conflict may occur because of two factors: physicians often fail to diagnose and report OA, and irritant exposures may increase the risk the risk of asthma without causing cases that meet the clinical definition. Both factors have important implications for proper treatment and prevention of asthma in adults. We propose to resolve this conflict. We will identify the role of irritant exposure in adult- onset asthma by simultaneously using both clinical and case control methods in a community-based perspective study of asthma incidence. The study cohort will be a typical US working population enrolled in an HMO. Additional benefits of the study design are the opportunities to validate a questionnaire for exposure assessment and for detection of work-related asthma. Specifically, we will: 1) Investigate incident cases in a cohort of over 80,000 adults over 3 years and determine the proportion that meet a clinical definition of OA; 2) Use a nested case-control study to determine the incidence of all asthma by occupation and workplace exposure; 3) Determine whether clinical OA accounts for the excess incidence of adult-onset asthma associated with workplace exposure to sensitizers and irritants; 4) Prospectively follow asthmatics for two years after diagnosis to determine the impact of adult-onset asthma on lung function, employment, income, and quality of life, and to determine whether prognosis differs for clinical OA and for asthma associated with workplace irritant exposure; 5) Test an intervention designed to increase appropriate clinical diagnosis, and thus secondary prevention of OA. This study will lead to improved methods of identifying asthma caused by workplace exposure and thus to a broader understanding of OA that will be an essential foundation for future efforts at primary prevention.